In optical recording, an increase in the information density on the optical record carrier must be accompanied by a smaller radiation spot for scanning the information. Such a smaller spot can be realised by increasing the numerical aperture (NA) of the objective system used for focusing a radiation beam in the scanning device on the record carrier. Examples of optical record carriers include CD's (Compact Discs) and DVD's (Digital Versatile Discs).
The radiation spot can never be a perfect point, due to diffraction limits. However, in such optical recording systems, it is desirable that the spot is limited by diffraction, and not by the effect of aberrations. Typically, an allowance in the root mean square of the optical path difference (OPD) of approximately 0.07λ, (where λ is the wavelength of the relevant radiation beam) in total is allowed for wavefront aberrations, such that the system is diffraction limited. It can be convenient to express the OPD in mλ (where 0.001 λ=1 mλ). A budget of approximately 30–40 mλ of aberration is permitted for the total wavefront aberration of the objective lens system from this total allowance, of which 15 mλ is allowed for the effects of coma. Other portions of the recording system and effects (e.g. temperature, wavelength error, misalignment of other components) will contribute to the total amount of aberration.
Many optical recording (and optical reading) systems have one or more additional beam spots focused on the optical record carrier adjacent to the central beam spot used for writing to (or reading from) the record carrier. Such additional spots are typically utilised to provide information on the positioning of the scanning (e.g. reading or writing) radiation spot on the record carrier. One example of such a system is the “three-spots push-pull” system, which has two auxiliary spots, one on either side of the main reading or writing spot. These additional spots are used to ensure that the central spot is in focus and in the desired position (e.g. on the desired track) of the carrier. The main spot is normally arranged on the optical axis of the objective system, and so it will be appreciated that the radiation beams forming the neighbouring spots will enter the objective lens system obliquely.
When assembling optical systems, it is important that the objective lens must not only be positioned correctly, but also orientated (i.e. tilted) correctly with respect to both the optical axis of the optical system and to the record carrier, so as to minimise the effect of aberrations, including coma.
Preferably, the optical axis of the objective lens system is aligned with the optical axis of the optical system (i.e. the collimated radiation beam), and perpendicular to the surface of the record carrier.
If the optical axis of the radiation beam incident on the record carrier is not normal to the record carrier, then coma will be induced in the beam as the beam passes through the transparent covering layer. Many optical recording carriers are not completely flat, but may be warped. Thus, as the radiation beam scans across the surface of the record carrier, it will encounter different areas of the surface which are tilted away from the preferred orientation. In order to compensate for the coma arising from such different tilts of the record carrier (e.g. a range of disk tilt), it is known to actively tilt the objective lens using an actuating unit.
Conventional lens designs are limited to a relatively small range of tilt compensation. Three-spots push-pull systems are particularly limited, due to the coma intrinsically arising due to the auxiliary spots entering the objective lens system obliquely even when the optical axis of the central scanning beam is arranged on the optical axis of the objective system.
It is an aim of embodiments of the present invention to provide an optical system which improves the range of disk tilt compensation that can be achieved by tilting the objective lens system.